STORAGE BATTERIES. 247 



Efficiency. The energy efficiency of a storage battery is the 

 ratio of the energy delivered by the cell during discharge to the 

 energy delivered to the cell during charge. The efficiency 

 ranges from 70 to 95 per cent, according to the conditions of use 

 of the battery. If the battery is discharged to 1.8 volts per cell, 

 then completely charged, then discharged to 1.8 volts per cell, 

 and so on, the efficiency varies from about 70 per cent, with high 

 rates of charge and discharge to about 80 per cent, with low 

 rates of charge and discharge. If the battery is charged and 

 discharged through a very narrow range during short intervals 

 of time, for example if a battery is charged for one minute then 

 discharged for one minute repeatedly, its efficiency may be as 

 high as 93 or 94 per cent. 



Rates of charge and discharge. The current delivered by a 

 storage cell, or the current used in charging it, measures its rate * 

 of discharge or charge. Thus one may properly speak of a ten- 

 ampere rate of charge or discharge. When a storage cell is 

 discharged at a high rate the voltage of the cell tends to fall off 

 rapidly, partly on account of the large Ri drop in the cell, but 

 chiefly because of the fact that the rapid absorption of the sul- 

 phuric acid in the formation of lead sulphate makes the acid in 

 the pores of the active material very dilute. On the other hand, 

 when a cell is charged at a high rate the voltage tends to rise 

 excessively. 



Furthermore, in rapid charging or discharging the tendency is 

 for the surface layers, only, of the active material to take part in 

 the voltaic action, so that the ampere-hour capacity of the cell is 

 reduced, and the rapid and non-uniform expansion and contraction 

 of the active material tends to cause disintegration. 



106, Examples of storage battery grids. There are two general 

 processes for making storage battery grids, namely : (a) The 

 Plante process and (^) the Faure process. In the Plante process 



* A discharge rate is usually specified in practice by giving the number of hours 

 required to discharge the cell at the given rate. Thus engineers speak of a two-hour 

 rate, a five-hour rate, etc. 



